1. Field of the Invention
The invention relates to a device for amplifying digital signals comprising an input electrode for receiving a digitally encoded signal, the device also comprising a subtraction unit for subtracting at least two signals from each other, the input electrode being coupled to a first input of the subtraction unit, and said device further comprising a cascade arrangement of a loop filter, a comparator and a switching amplifier, this cascade arrangement being coupled to an output of said subtraction unit, and said device also including an output electrode which is coupled to an output of the switching amplifier, said output of the switching amplifier also being coupled to a second input of the subtraction unit.
The invention further relates to a bridge amplifier comprising at least two such devices, and to an integrated circuit provided with such a device.
2. Description of the Related Art
Such a device is known from preprint 4448 (G6) of the 102.sup.nd convention of the Audio Engineering Society, held from Mar. 22 to Mar. 25, 1997. This publication is entitled "A Sigma-Delta Power Amplifier for Digital Input Signals". By means of such a device, a digital input signal, for example, a pulse-density-modulated audio signal, can be converted to an amplified output signal. To achieve this, the device comprises a switching amplifier which is controlled by a comparator. To reduce the sensitivity of this switching amplifier to variations, for example, in the supply voltage, the amplified output signal is fed back to the input. This feedback causes the control of the switching amplifier by the comparator to be influenced in such a manner that, in case of variations in the supply voltage of the switching amplifier, the output signal is corrected. This correction can be carried out, for example, in such a manner that the low-frequency contents of the output signal remains substantially the same. Such devices further include a loop filter which should provide for a maximum loop gain at low frequencies, while the loop gain at the sampling frequency should be below one so as to preclude oscillations. The latter condition can be met by constructing the device in such a manner that the high-frequency portion of the input current is larger than the high-frequency portion of the feedback current. To ensure that such a device operates in a stable manner, it is also necessary that the loop gain at a frequency close to the frequency at which the loop gain is equal to one is in accordance with a first-order characteristic. To achieve the above-mentioned requirements, use is generally made of a loop filter which ensures that the loop gain of the device at low frequencies is in accordance with a higher-order characteristic, so that a relatively high loop gain is obtained, and said loop filter also ensures that, from a certain transition frequency, said loop gain is in accordance with a first-order characteristic, the loop gain at the transition frequency being greater than one.
The device known from the above-mentioned publication includes a clocked loop filter and a clocked comparator. As a result, in the case of variations, for example, in the supply voltage, the output signal is corrected by adding or leaving out pulses. In the known device, adding or leaving out pulses causes quantization noise in the output signal, which leads to a decrease in dynamic range. This quantization noise can be reduced in a specific frequency range of the output signal by means of a relatively complex and expensive loop filter, so that an increase in dynamic range is obtained.